Since the exposed terminal surface of pn-junction formed on semiconductor substrate is sensible to the outer atmosphere, it is stabilized by coating it with various passivating materials. In Japanese Patent Application Kokai (Laid-Open) No. 8,469/75, there is disclosed a passivation technique by the use of a polyimide resin in place of an inorganic passivation material such as silicon dioxide, silicon nitride, alumina, glass or the like, and said technique is advantageous in that the stabilizing technique can be practised at a low temperature. The polyimide resin disclosed in said patent specification is that obtained from pyromellitic acid dianhydride, benzophenonetetracarboxylic acid dianhydride, diamino diphenyl ether and diamino monocarboxamide diphenyl ether. And, it is mentioned to pre-treat the semiconductor substrate and the like with an aminosilane coupling agent for the sake of improving the adhesion between the polyimide resin and the semiconductor substrate and the like.
In Japanese Patent Application Kokai (Laid-Open) Nos. 6,279/75 and 6,280/75, there is disclosed a technique for forming a bonding layer of polyimide or the like between encapsulation resin and semiconductor integral circuit chip or between the undercoat formed on the chip and the chip to decrease the failure of IC device due to the invasion of water from the outer atmosphere. In U.S. Pat. No. 4,079,511, there is also disclosed a semiconductor device which is covered with a sealant so that water does not reach the integral circuit chip.
Further, in U.S. Pat. Nos. 4,001,870, 4,017,886, 4,040,083 and 4,060,828, there is disclosed a technique in which a polyimide resin is used as a material for the insulating film in multilevel metal layers. And, in U.S. Pat. No. 4,040,083, the semiconductor chip is pre-treated with an aluminum chelate compound for the purpose of improving the adhesion between the polyimide resin and SiO.sub.2 film and improving the water resistance of the semiconductor device.
On the other hand, in order to prevent the soft errors caused by the .alpha.-rays emitted by the radioactive impurities, i.e. uranium and thorium, present in the encapsulation material in semiconductor memory device having a high integration density, there is a technique for forming an .alpha.-rays shielding layer in the circuit side of semiconductor memory chip, and in U.S. patent application Ser. No. 222,612 filed on Jan. 5, 1981 entitled "SEMICONDUCTOR MEMORIES AND PROCESS FOR PRODUCING THE SAME", there is disclosed a resin encapsulated semiconductor memory device in which a polyimide resin having a much minimized content of uranium and thorium is used as the shielding material.
According to these prior techniques, it is acknowledged that the most important problem in the resin encapsulated semiconductor device as compared with the case of ceramic encapsulation or glass encapsulation is that the resin has a nature of allowing the permeation of water and that, due to the difference in thermal expansion coefficient between the resin and the semiconductor substrate and invasion of water into the interface therebetween, a gap is formed between them which causes the permeation of water. For this reason, it is usual to treat the interstices between, for example, an undercoat resin and a semiconductor chip or metallic wire with a coupling agent.
As is known, there is a general tendency to require high water resistance of the resin materials used in the field of semiconductor devices. One of its examples is the extensive use of silicone resin as passivation material. However, silicone resin has a problem in that it is inferior in bonding strength to semiconductor substrates, metals and encapsulation resins.
Various polyimide resins and particularly polyimide-isoindoloquinazolinedione resin have become extensively employed in the field of integrated circuit because they have excellent characteristics such as heat resistance, insulating properties, coating film formability and the like and have bonding strength to semiconductor chips and metallic films.
However, the present inventors studied on semiconductors not only with the above-mentioned polyimide-isoindoloquinazolinedione resin but also with many kinds of polyimide resins to find that some of them developed failure after a longterm severe test for water vapor resistance. In order to research into the cause of the failure, the inventors studied on the water vapor resistance of encapsulation resins, bonding properties between encapsulation resins and polyimide film as an undercoat or a stabilizing film, and the bonding strength between the polyimide film and the semiconductor chip or metallic films. As a result, it was found that the most important cause of the failure was water absorption of the polyimide resin itself. It has hitherto been considered that permeation of water is impossible if the conductor chip and metallic film are solidly and tightly bonded to the polyimide film. However, the fact that failure due to water cannot be prevented even if the conductor chip is pre-treated with coupling agent so sufficiently leads to a conclusion that the problem cannot be solved by merely improving the bonding strength between the semiconductor chip and the polyimide resin. That is, this failure is considered ascribable to that polyimide resin absorbs water and acts as a storehouse of water.